ニコニコ動画のニコ生で、『おぅふ』って見かけます。どーゆう意味ですか?
Here is a funny exercise $$\sin(x - y) \sin(x + y) = (\sin x - \sin y)(\sin x + \sin y).$$ (If you prove it don't publish it here please). Do you have similar examples?